*{
    margin: 0;
    padding: 0;
}

.box {
    position: relative;
    width: 400px;
    height: 400px;
    margin: 100px auto;
    background-color: pink;
}

.son {
    position: absolute;
    width: 200px;
    height: 200px;
    left: 50%;
    top: ;
    transform: translateX(-50%);
    background-color: skyblue;
}